J. P. Toennies

The van der Waals potentials between all the rare gas atoms from He to Rn

The interatomic van der Waals potentials for all the possible 21 homogeneous and heterogeneous pairs of rare gas atoms including radon are determined using the Tang–Toennies potential model and a set of previously derived combining rules. The three dispersion coefficients and the two Born–Mayer parameters needed for calculating the potential curves are listed.

Successive capture and coagulation of atoms and molecules to small clusters in large liquid helium clusters

The multiple pickup of up to 30 foreign particles by large (N≳103 atoms) liquid helium clusters and the formation of clusters of the foreign particles within the helium cluster has been observed in a molecular beam experiment for Ar, Kr, Xe, H2O, and SF6. Evaporation of helium atoms due to the pickup process has been measured quantitatively by two methods. The measured apparent cross sections for capture and coagulation of these particles are significantly smaller than the total scattering cross sections of the clusters. After electron impact ionization the size distribution of the foreign particle cluster ion fragments can be well described by a Poisson distribution. This is interpreted as indicating negligible fragmentation of the coagulated foreign particle clusters. The measured coagulation cross sections are smaller than the capture cross sections. This is explained through a model which involves only partial coagulation of the foreign particles. The observed processes allow the preparation of foreig...

A new scattering deflection method for determining and selecting the sizes of large liquid clusters of 4He

We introduce a new small-angle (θ<20 mrad) scattering deflection method to determine the size distribution of large liquid helium clusters formed by condensation in a supersonic expansion. The size distributions can be well represented by a log—normal model with mean sizes varying between 3×103 and 104 atoms depending on the source conditions. This deflection method can also be employed to size-select helium clusters.

Erratum: A simple theoretical model for the van der Waals potential at intermediate distances. I. Spherically symmetric potentials

A simple potential model is presented, which uses available ab initio calculated short range SCF Born–Mayer parameters and the perturbation theoretical dispersion terms for the long range potential. In the intermediate region two corrections are shown to be necessary to take account of the divergence of the dispersion expansion and the influence of electron overlap. These corrections can be predicted from the known asymptotic parameters and atomic properties. The resulting potential model, which contains only seven potential parameters, is shown to predict the van der Waals potential parameters R0[V (R) =0], Rm (well location), and e (well depth) for six rare gas–rare gas atom combinations and six open shell–rare gas atom combinations. The predicted potential shape for Ar–Ar is in excellent agreement (≲3%) with the best experimental determination. The model is used to predict Born–Mayer parameters from measured values of Rm and e.A simple potential model is presented, which uses available ab initio calculated short range SCF Born–Mayer parameters and the perturbation theoretical dispersion terms for the long range potential. In the intermediate region two corrections are shown to be necessary to take account of the divergence of the dispersion expansion and the influence of electron overlap. These corrections can be predicted from the known asymptotic parameters and atomic properties. The resulting potential model, which contains only seven potential parameters, is shown to predict the van der Waals potential parameters R0[V (R) =0], Rm (well location), and e (well depth) for six rare gas–rare gas atom combinations and six open shell–rare gas atom combinations. The predicted potential shape for Ar–Ar is in excellent agreement (≲3%) with the best experimental determination. The model is used to predict Born–Mayer parameters from measured values of Rm and e.

Mass spectra and time‐of‐flight distributions of helium cluster beams

Liquid helium clusters are produced by expanding gaseous 4 He into a vacuum from a cold source with temperatures between 5 and 20 K at stagnation pressures from P0 =8 to 20 bar and are studied by time‐of‐flight (TOF) and mass spectrometry. At low temperatures, T0 <12 K, the mass spectra show several anomalies which can be attributed to pick‐up of residual gases. At T0 <10K, there is evidence for a very intense peak at m=16 amu which is attributed to He+4 . Depending on the temperatures, the TOF spectra reveal ions with three different velocities. These TOF observations are analyzed using isentropic lines in the known phase diagram of 4 He, which take into account deviations from ideal gas behavior. Three qualitatively different expansion regimes are identified: (I) the expansion proceeds through a region on the high temperature side of the critical point, (II) the expansion passes through or near the critical point, and (III) the expansion passes through a region on the low temperature side of the critica...

A molecular beam study of the evaporation of water from a liquid jet

A method to maintain a clean surface of a liquid in a high vacuum is described. Using a very thin and fast liquid jet it is not only possible to prevent freezing of the liquid but also to reduce the number of collisions between evaporating molecules to negligibly small values. Thus many of the standard, vacuum dependent, particle probing techniques for solid surfaces can be used for studies of rapidly vaporizing, high vapor pressure liquids. In a first molecular beam investigation we have used time-of-flight analysis to measure the velocity distribution of H2O molecules vaporizing from thin jets of pure liquid water. The experiments were carried out for liquid jet diameters between 50 and 5 µm. In this range the expanding vapor is observed to undergo the transition to the collision-free molecular flow regime. From the measured velocity distributions the local surface temperature is determined to be less than 210 K. This appears to be the lowest temperature ever reported for supercooled liquid water.

Orientational ordering of two-dimensional ice on Pt(111)

Two highly ordered, epitaxially rotated phases of bilayer ice are observed on Pt(111) in high resolution helium atom diffraction. Analysis of helium diffraction patterns shows that the two phases differ slightly in their density and alignment with respect to the surface. The lack of any isotope effect for both phases indicates that the ice bilayers have structures and hydrogen bond lengths very similar to bulk ice.

Excitation and ionization of 4He clusters by electrons

Clusters are produced by expanding high pressure (P0≤20 bar), low temperature (T0≥5 K) helium gas through a 5 μm nozzle into a vacuum. The neutral beam time‐of‐flight distribution has three peaks which we associate with distinct groups of large and small clusters, and atoms. The beam is ionized by electron impact and the resulting time resolved charged fragment mass distribution reveals in addition to previously observed anomalies (‘‘magic numbers’’) a new strong He+4 signal at high source pressures and low temperatures. The dependence of the various charged and neutral metastable fragment currents on the bombarding electron energy reveals that each has a unique appearance potential. A comparison with the calculated energy required for an electron to create various electronic excitations in the interior of a large cluster indicates that the production and dynamical evolution of metastable 3S1 atomic and a 3Σ+u molecular excitations plays a significant role in the formation of charged fragments from large ...

Determination of Atom-Surface van der Waals Potentials from Transmission-Grating Diffraction Intensities

Molecular beams of rare gas atoms and

Evidence for Quantum Size Effects Observed by Helium Atom Scattering during the Growth of Pb on Cu(111)

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